The Power of Breathing for Stress Relief
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The human heart, a marvel of nature's engineering, is predominantly composed of a specialized type of striated muscle tissue known as the myocardium. This tissue is comprised of cells referred to as cardiac myocytes, which are integral to the heart's function. However, a mere 1% of these myocardial cells play a vital role in the spontaneous generation of the contractile stimulus, regulating the heart rate.
The properties of myocardial cells share similarities with other types of muscle tissue, including smooth and voluntary striated muscle tissues. Just like smooth muscles, which regulate the involuntary movements of internal organs, cardiac cells are small and contain a single nucleus.
The myocardium, akin to skeletal muscle, has its functional unit in the sarcomere. It also shares striation and contraction mechanisms with skeletal muscle. A unique characteristic of the heart is the way individual cells connect, often ending with visible branches. This allows cardiac myocytes to connect at their ends through structures called gap junctions, facilitating both electrical and mechanical coupling. This connection allows the transfer of contractile tension from one cell to another, making the cells contract almost simultaneously. This synchronization allows the functioning heart muscle to operate like a single large cell.
Mitochondria, the powerhouse of cells, occupy about a third of a cardiac myocyte's volume. The abundant presence of mitochondria enables the cardiac muscle to extract oxygen from the blood more efficiently than other tissues. Even at rest, the heart extracts approximately 70-80% of the oxygen contained in arterial blood, more than double the amount extracted by other body cells. During intense physical effort, the heart's increased energy demands can only be met through an increase in blood flow within the coronary arteries.
The heart is a highly vascularized organ, with each of its cells supplied by at least one capillary. Oxygen is fundamental for the optimal functioning of the heart muscle. Unlike other striated muscles, the heart has a limited ability to draw energy from anaerobic processes. If deprived of oxygen, the cardiac muscle cells can die within minutes, leading to a condition known as infarction. This can be fatal or cause serious deficits, as damaged myocardial cells cannot regenerate.
The myocardium, which is the muscular layer of the heart wall, varies in thickness between different sections of the heart. Specifically, the myocardium of the atria is thinner compared to that of the ventricles. This difference in thickness is particularly notable in the left ventricle.
The walls of the left ventricle are significantly thicker than those of the atria and even the right ventricle. This structural adaptation is important because the left ventricle performs the demanding task of pumping oxygenated blood into the systemic circulation. During the systole phase of the cardiac cycle, the left ventricle contracts forcefully, generating the high pressure needed to propel blood through the aorta and into the arteries that supply vital organs and tissues throughout the body.
In contrast, the right ventricle, which pumps blood to the pulmonary circulation, and the atria, which receive blood returning to the heart, have less muscular walls. They deal with lower pressure circulations, which require less forceful contractions.
Like all muscles, the heart contracts in response to an electrical stimulus. However, the myocardium can autonomously generate this stimulus thanks to a structure known as the sinoatrial node. This node, rich in pacemaker cells, propagates waves of electrical impulses that reach the cardiac muscle cells, generating and regulating the heartbeat. This process is not random, but is regulated to generate the rhythmic alternation of contraction (systole) and relaxation (diastole), known as the cardiac cycle.
Though the heart has autonomous contractile capacity, its activity is influenced by the nervous system, which regulates the heart rate based on the body's changing needs. The heart receives inputs from the orthosympathetic system, which accelerates the heartbeat, and from the vagus nerve, part of the parasympathetic system, which slows down the myocardial excitation rate.
To fuel its contractions, the heart primarily utilizes the oxidation of fatty acids. Beyond fatty acids, the heart possesses the versatile capability to metabolize other substrates depending on the body's physiological state and available resources. These alternative sources include glucose, which becomes particularly important during high-intensity activities or in conditions such as diabetes; lactic acid, which is predominantly utilized after being transported from skeletal muscles during vigorous exercise; and ketone bodies, which become a key energy source during prolonged periods of fasting or low-carbohydrate diets. This metabolic flexibility ensures that the heart can maintain its vital function under various nutritional states.